Recently, to adapt to use under stricter sliding conditions in bearings used in automobiles, construction machines, agricultural machines, and the like, there has been a demand for the development of a copper-alloy-based sliding material as a bearing material having high load resistance as well as superior adhesion resistance and seizing resistance. In addition, to reduce environmentally burdening substances, there is a demand for the development of a copper-based sliding material that does not contain lead.
Among conventional materials for bearings in the travel components of a construction machine, a Cu—Sn—Pb (lead bronze)-based material, e.g., Cu-10Sn-10Pb (LBC-3) has been used as a material having improved seizing resistance and satisfactory sliding characteristics (see Japanese Laid-Open Patent Application No. 2002-295473 (paragraph 0003), for example).
However, since the aforementioned lead bronze-based material contains lead, it is believed that in cases in which oil leaks from the bearings, Pb also flows out with the oil into the environment, and the burden on the environment increases. The need for environmental concern has particularly increased of late, and there is a demand for a sliding material that does not contain Pb. Therefore, Ag and Bi have been proposed as materials replacing Pb (see Japanese Laid-Open Patent Application No. 2002-60869 (claims 1 and 2), for example).
Another consideration has been to provide an overlay of a resin or another nonmetal material as a sliding layer over the sliding surfaces, thereby ensuring the break-in characteristics and seizing resistance normally provided by Pb (see Japanese Laid-Open Patent Application No. 2004-307960 (paragraph 0002), for example).
Another proposal has been a copper-based sliding material wherein a large amount of graphite, molybdenum disulfide, or another solid lubricant is mixed into bronze, ensuring seizing resistance and high lubrication. However, this copper-based sliding material does not sinter well, and high mechanical strength cannot be ensured.
There is also a technique for dispersing hard particles such as a ceramic powder into a soft metal matrix whose Sn content has been adjusted, the intention being to improve the abrasion resistance and break-in characteristics (low coefficient of friction) needed for the sliding material. However, this sliding material has a drawback in that due to insufficient bond strength between the soft metal matrix and the hard particles as well as the large difference in hardness, a phenomenon occurs in which the hard particles flake off, and the flaked-off hard particles adversely affect the other shafts and instead reduce sliding performance.